Antigen recognition of T lymphocytes is mediated by highly diverse cell surface glycoproteins known as T cell receptors (TCRs). TCRs interact with a class of molecules known as superantigens (SAGs) which stimulate T cells. Superantigens, which may be of viral or bacterial origin, can induce profound immunological responses in the host such as toxic shock. However, no structural information is available on the interaction of SAGs with TCRs.